The present invention relates to microelectronics, and more particularly to assembling microelectronic components.
Microelectronic components have miniature circuits with features too small to be handled by humans. Examples of such components are semiconductor integrated circuits (ICs 110 in FIG. 1), interconnection substrates (e.g. 120), and their combinations (component 122 of FIG. 1 is a combination of components 110 and 120). Substrate 120 provides interconnections between different ICs 110 and/or between the ICs and other microelectronic components and other circuits. Examples of interconnection substrates are printed circuit boards (PCBs) and interposers; an interposer is an intermediate interconnection substrate, with other microelectronic components attached both to the top and the bottom of the interposer. In the example of FIG. 1, contact pads 110C of two ICs 110 are attached to contact pads 120C of substrate 120. Substrate 120 has interconnect lines 150 which interconnect the contact pads 120C in a desired pattern. The attachments of contact pads 110C to contact pads 120C are shown at 140; these can be solder, adhesive, or thermocompression attachments (in thermocompression, the contact pads are attached without solder, adhesive, or any other bonding agent). Contact pads 110C could also be connected to contact pads 120C by discrete wires, but discrete wires undesirably increase the size of the assembly and the length of the connections. Short lengths are preferred to reduce power consumption and parasitic capacitances and inductances and to increase the operating speed.
Each IC 110 and substrate 120 may include densely packed circuits with hundreds or thousands of contact pads 110C and 120C per square inch or square centimeter. Therefore, the contact pads and the connections 140 must be small. However, small connections can easily brake due to stresses arising from thermal expansion and contraction. In order to strengthen the connections 140, the surrounding spaces are filled with underfill (UF) 130. Underfill 130 is an adhesive that glues the components 110 to components 120 and thus relieves some of the stress on connections 140.
In older technologies still in use today, the underfill 130 is introduced after attachment of ICs 110 to substrate 120; the underfill is introduced in liquid form at the IC periphery, and is drawn to the IC underside by capillary forces. Then the underfill is cured to solid state. Hopefully, the underfill will have no voids. However, the capillary process and the subsequent curing take much time, and place stringent requirements on the underfill material and the process conditions, especially if the IC 110 is large. Void-free capillary underfilling can be a demanding procedure.
Another option is pre-applied underfills, i.e. the underfills applied before attaching ICs 110 to substrate 120. FIG. 2A shows liquid (viscous but flowing) underfill 130 pre-applied on IC 110 or substrate 120 and squeezed as the IC is being placed on the substrate. Contact pads 120C have been “bumped” with solder 140 before the underfill process. FIG. 2B shows the IC's contact pads 110C and 120C being joined by solder connections 140. At this stage, the solder is reflowed (melted) to bond the contact pads 110C to contact pads 120C, and underfill 130 is cured.
This process requires strict control of the underfill deposition: if the UF layer is too thick, then voids (bubbles) are likely to form in the underfill. Also, undesirably, any excess of the pre-applied underfill flows out from under the IC and may affect adjacent circuitry.
Another type of pre-applied underfill is non-conductive film (NCF), applied to either IC 110 (as in FIG. 3A) or to substrate 120 in liquid form and partially cured before attachment of components 110, 120 to each other. Alternatively, NCF can be applied in dry (solid) form, e.g. by a heat roller; the heat makes NCF to flow slightly during application and to cover the IC 110 without voids.
In the example of FIG. 3B, IC 110 with NCF 130 is attached to the substrate 120. This process is conducted at an elevated temperature and a sufficient pressure (illustrated by the force F) to soften NCF 130 so that the contact pads 110C and 120C penetrate the NCF and bond together (e.g. by thermocompression, or by solder, not shown; the solder can be formed on contact pads 110C before they are covered by the NCF).
A major challenge in this process is to provide low contact resistance, i.e. low electrical resistance at the juncture 350 of contact pads 110C and 120C. Since NCF is dielectric, and the juncture area 350 is small, even minute NCF residue in the area 350 can significantly lower electrical conductivity. If the IC has hundreds or thousands of contact pads 110C, even a single bad connection (between a single pair of contact pads 110C and 120C) can make the assembly inoperable. Therefore, it has been proposed to arrange the NCF so that the contact pads 110C are exposed before the attachment of IC 130 to substrate 120 (as in FIG. 3C). See U.S. pre-grant patent application no. 2011/0237028 (Hamazaki et al., Sep. 29, 2011), and U.S. Pat. No. 6,916,684 (Stepniak et al., Jul. 12, 2005).